1. Field of the Invention
The present invention relates to a constant power circuit for controlling a load such that it dissipates power at a constant power level during an overpower state of the load. More particularly, the constant power circuit of the present invention limits to a constant safe level the power dissipated by an electronic load during transient and steady-state conditions of the electronic load.
2. Description of the Prior Art
An electronic load of the type to which the present invention is directed is an electronic circuit which generally comprises channeled FETs for dissipating large amounts of power applied to inputs thereof. Such an electronic load has several independently programmable modes and is typically applied to the output of a power source for checking the response characteristics of the power source. For example, the power source may be tested by connecting its output across the electronic load and independently programming the electronic load to have a specified constant current mode or constant voltage mode or to have a specified constant of proportionality for a mode in which the current is linearly proportional to the voltage. By so selecting a particular mode and load demand values (constant current levels and the like), a user of the electronic load can control the nature of the load and hence the amount of power dissipated thereby so that the power source characteristics can be monitored for different load demands.
Prior art electronic load circuits of the type just described typically provide negative feedback control to maintain steady-state conditions for measurement. However, a problem occurs in such devices of the prior art in that when the electronic load is switched by the user from one regulatory mode to another or from one level to another the power dissipated by the electronic load may temporarily exceed the power limit of the electronic load circuitry. In other words, although the voltage, resistance and/or current are maintained relatively constant by feedback control, the power dissipated by the electronic load still may be too great for the circuitry of the electronic load, thereby causing overheating of its components. As a result, disconnection of the electronic load from the power source or some other load control measure has to be taken.
Disconnection of the electronic load during an overpower condition is not desirable since it is necessary that the electronic load remain connected to the power source throughout testing procedures for accurate test measurements to be taken. Furthermore, it is desirable to monitor the power source characteristics when the electronic load is switched from a constant current to constant voltage or constant resistance mode and the like or when the level of the constant current or constant voltage is changed. However, some overpower protection must be provided in such cases to prevent the electronic load circuitry from being damaged when the electronic load is caused to dissipate more power than its circuitry can safely handle.
One approach to the above problem has been to provide no overpower protection at all. Rather than provide such overpower protection, thermal shutdown of the electronic load circuit occurs when an overpower condition is detected. However, such an approach has the obvious problem that by the time the thermal sensor determines that an overpower condition has occurred the transistors of the electronic load circuit may be irreparably damaged.
Another approach to the above problem has been to provide "soft overpower" protection. In this technique, the voltage and current passing through the electronic load are measured and the product of the voltage and current is calculated by a microcomputer. The microcomputer then takes appropriate action to prevent the electronic load from being driven in an overpower condition. However, such a technique is not preferable since the circuitry can be quite expensive and since a substantial amount of processing time is often required for the overpower condition to be determined and then corrected. A faster and less expensive solution is desired.
An analog multiplier has been used to shorten the processing time for determining the overpower condition. As shown in FIG. 1, for example, an analog multiplier has been used to generate a voltage which represents a product of signals that are proportional to the detected current and voltage. The resulting power level is then limited by a control transistor which is controlled to regulate the power dissipated. However, such a circuit has the limitations that a separate linear negative feedback control loop responsive to the current and voltage is provided for protecting from an overpower condition, as shown in FIG. 1. Due to the linear nature of this control loop, it must be compensated with respect to the customer's power source characteristics to account for such problems as differing impedances at the output of the power source. Such compensation tends to be expensive and requires a significant amount of calibration time since manual adjustment of the analog multiplier is often necessary.